sv_grow: performance improvement for short strings

Empty COW strings with CUR=0 ended up allocated as LEN=10.
Now they are rounded up to 4 or 8.

Timings:
+0   16.394324103 0.27%
+2   16.114379842 0.01%
+4   16.305622265 1.03%
+8   16.337438609 1.30%
+10  16.675009468 0.59%
with LD_LIBRARY_PATH=`pwd` perf stat -r2 ./perl t/harness t/op/*.t

+2 was consistently the best number, and +10 the worst.

1 files changed, 2 insertions, 2 deletions

diff --git a/sv.c b/sv.c
index 5a77d6bd43..2940942fc6 100644
--- a/sv.c
+++ b/sv.c
@@ -1553,7 +1553,7 @@ Perl_sv_grow(pTHX_ SV *const sv, STRLEN newlen)
 
 #ifdef PERL_NEW_COPY_ON_WRITE
     /* the new COW scheme uses SvPVX(sv)[SvLEN(sv)-1] (if spare)
-     * to store the COW count. So in general, allocate one more byte than
+     * to store the CowREFCNT. So in general, allocate one more byte than
      * asked for, to make it likely this byte is always spare: and thus
      * make more strings COW-able.
      * If the new size is a big power of two, don't bother: we assume the
@@ -1569,7 +1569,7 @@ Perl_sv_grow(pTHX_ SV *const sv, STRLEN newlen)
 
     if (newlen > SvLEN(sv)) {		/* need more room? */
 	STRLEN minlen = SvCUR(sv);
-	minlen += (minlen >> PERL_STRLEN_EXPAND_SHIFT) + 10;
+	minlen += (minlen >> PERL_STRLEN_EXPAND_SHIFT) + 2;
 	if (newlen < minlen)
 	    newlen = minlen;
 #ifndef PERL_UNWARANTED_CHUMMINESS_WITH_MALLOC